How Do You Catch a "Cold" in the Tropics?

The weather report in KL is basically the same every day- low temperature 24 degrees C (75 degrees F) and high temperature 31 degrees C (89 degrees). I am generally a pretty healthy person. Back home,I ususally get a cold around the end of January or the beginning of February when Lubbock's winter is at its worst (that is also the time when I am usually lecturing about "photosynthesis"- maybe that is what makes me sick). I have to wonder- "how can you catch a cold when the temperature never gets below 75 degrees?!? (and I haven't even thought about photosynthesis recently!)